The present invention relates to a catalyst for purifying an exhaust gas, a process for producing the same, and a method using the catalyst for purifying an exhaust gas, in particular, to an NOx storage and reduction type catalyst which can efficiently purify nitrogen oxides (NOx) in an exhaust gas which contains oxygen excessively in an amount more than necessary for oxidizing carbon monoxide (CO) and hydrocarbons (HC) which are contained in the exhaust gas, a process for producing the same and a method for purifying the exhaust gas by using the catalyst.
Conventionally, as a catalyst for purifying an automobile exhaust gas, a 3-way catalyst has been employed which carries out the oxidation of CO and HC and the reduction of NOx simultaneously to purify an exhaust gas. With regard to such a catalyst, for example, a catalyst has been known widely in which a loading layer comprising xcex3-alumina is formed on a heat-resistant support, such as cordierite, and a noble metal, such as Pt, Pd and Rh, is loaded on the loading layer.
By the way, the purifying performance of such a catalyst for purifying an exhaust gas depends greatly on the air-fuel ratio (A/F) of an engine. For example, when the air-fuel ratio is large, namely on a lean side where the fuel concentration is lean, the oxygen amount in the exhaust gas increases so that the oxidation reactions of purifying CO and HC are active, on the other hand, the reduction reactions of purifying NOx are inactive. Conversely, for example, when the air-fuel ratio is small, namely on a rich side where the fuel concentration is high, the oxygen amount in the exhaust gas decreases so that the oxidation reactions are inactive and the reduction reactions are active.
Whilst, in order to suppress the recent global warming, it is required to control the CO2 emission in automobiles. In order to meet the requirement, the lean burn is effective in which the burning is carried out in an oxygen-rich lean atmosphere having a large air-fuel ratio, engines, which are appropriate for the lean burn, have been made practicable. However, when purifying the exhaust gas emitted from the lean burn engines, there arises a problem in that it is difficult to purify the NOx as aforementioned.
Hence, an NOx storage and reduction type catalyst has been proposed in which an alkaline-earth metal and Pt are loaded on a porous support, such as alumina (Al2O3), (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, etc.). In accordance with this catalyst, since the NOx are absorbed in the alkaline earth metal, serving as the NOx storage member, and since they are reacted with the reducing components, such as HC, and are purified, it is possible to control the emission of the NOx even on the lean side.
In the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, it is believed that barium, for example, is loaded as the carbonate, and the like, on the support, and it reacts with NOx to generate barium nitrate (Ba(NO3)2) in a lean atmosphere, thereby storing the NOx. And, when the exhaust gas is in the range of from the stoichiometric point to the rich atmosphere, the stored NOx are released and are reacted with the reducing components, such as HC and CO, and are thereby reduced. And, in order to enhance the NOx conversion by carrying out these reactions of the NOx storage member efficiently, an engine control method has been developed, in which the air-fuel ratio is usually controlled on an oxygen-excessive lean side and it is intermittently controlled in the range of from the stoichiometric point to the rich atmosphere in a pulsating manner, and has been put into practical applications.
In accordance with this engine control method, since the used amount of the fuel is less on the lean side, the emission of the CO2 is suppressed, and the NOx are stored in the NOx storage member. Accordingly, the emission of the NOx is suppressed on the lean side as well. And, the stored NOx are released in the range of from the stoichiometric point to the rich side, and are reacted with the reducing components, such as HC and CO, by the catalytic action of Pt, and so on. Therefore, a high NOx purifying capability is exhibited as a whole.
However, in the exhaust gas, SO2 is contained which is generated by burning sulfur (S) contained in the fuel, it is further oxidized to SO3, by the noble metal in an oxygen-rich atmosphere. Then, they are easily reacted with the barium, etc., to generate sulfites and sulfates, and it is understood that the NOx storage member is thus poisoned and degraded. This phenomenon is referred to as sulfur poisoning. Moreover, the porous support, such as alumina, has a property that it is likely to adsorb the SOx, and there is a problem in that the aforementioned sulfur poisoning is facilitated.
And, when the NOx storage member is turned into the sulfites and the sulfates, it cannot store the NOx any more, and, as a result, there is a drawback in the aforementioned NOx storage and reduction type catalyst in that the NOx purifying ability decreases after a durability test.
Moreover, since TiO2, which is an acidic oxide, does not adsorb SO2, it was thought of using a TiO2 support, and an experiment was carried out. As a result, SO2 was not adsorbed by the TiO2 and flowed downstream as it was, since only the SO2 which contacted directly with the noble metal, was oxidized, it was revealed that the sulfur poisoning occurred to a lesser extent. However, in the TiO2 support, a drawback was revealed that the initial activity was low. This reason is believed that the NOx purifying capability of the NOx storage member decreases because TiO2 reacts with the NOx storage member to form composite oxides (BaTiO3, etc.) in the temperature range of the exhaust gas.
Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 6-327,945, it is proposed to use a support in which Al2O3 is mixed with a composite oxide, such as a Baxe2x80x94Ce composite oxide and a Baxe2x80x94Cexe2x80x94Nb composite oxide. In addition, in Japanese Unexamined Patent Publication (KOKAI) No. 8-99,034, it is proposed to use at least one composite support selected from the group consisting of TiO2xe2x80x94Al2O3, ZrO2xe2x80x94Al2O3 and SiO2xe2x80x94Al2O3.
By thus using the support in which the composite oxide is mixed, or by using the composite support, the NOx storage member is inhibited from the sulfur poisoning, and the NOx purifying capability after durability is improved.
However, a further reduction of the CO2 emission is required against the recent issue of the global warming, and the lean burn driving range tends to increase. Accordingly, since the sulfur poisoning tends to further increase, and since the exhaust gas emission control tends to be further strengthened, the exhaust gas purifying catalysts are required to furthermore improve their durability.
The present invention has been developed in view of the aforementioned circumstances, and it is an object of the present invention to make an exhaust gas purifying catalyst, in which the sulfur poisoning of the NOx storage member can be further inhibited, and which can maintain a high NOx conversion even after a durability test.
A characteristic of an exhaust gas purifying catalyst according to the present invention, solving the aforementioned assignments, is that the catalyst comprises: a support including a porous oxide including TiO2 at least and ZrO2 on which Rh is loaded in advance; an NOx storage member including at least one member selected-from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support; and a noble metal including at least one member selected from the group consisting of Pt, Pd and Rh and loaded on the support.
In the aforementioned exhaust gas purifying catalyst, it is preferred that the porous oxide includes a composite oxide of Al2O3 and TiO2.
Further, a characteristic of a process for producing an exhaust gas purifying catalyst according to the present invention is that the process comprises the steps of: a step of forming a loading layer on a substrate by using a slurry which includes a porous oxide including TiO2 at least and ZrO2 with Rh loaded in advance; and a step of loading a noble metal including at least one member selected from the group consisting of Pt, Pd and Rh an NOx storage member including at least one member selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements.
Furthermore, a characteristic of an exhaust gas purifying method according to the present invention is that, by using an exhaust gas purifying catalyst, which comprises a support including a porous oxide including TiO2 at least and ZrO2 on which Rh is loaded in advance; an NOx storage member including at least one member selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support; and a noble metal including at least one member selected from the group consisting of Pt, Pd and Rh and loaded on the support, NOx are stored in a lean exhaust gas, which is generated by burning an air-fuel mixture of an oxygen excess lean atmosphere; and the stored NOx are released and reduced in a rich exhaust gas, which is generated by burning an air-fuel mixture of from a stoichiometric point to a fuel rich atmosphere.
In the exhaust gas purifying catalyst according to the present invention, a support is constituted by a porous oxide including TiO2 at least and ZrO2 with Rh loaded in advance (hereinafter referred to as Rh/ZrO2), and an NOx storage member and a noble metal are loaded on the support. This Rh/ZrO2 exhibits a high function of reducing and purifying NOx, which are released from the NOx storage member in an exhaust gas atmosphere of from a stoichiometric point to rich, and the NOx purifying performance is improved sharply. Further, compared with Pt, Rh grows granularly less remarkably in a lean atmosphere. Therefore, the durability upgrades by the presence of Rh.
Further, by Rh, hydrogen of a high reducing power is generated from HC and H2O in the exhaust gas (steam reforming reaction), this hydrogen contributes greatly to the reduction of NOx and the elimination of SOx from the sulfites and sulfides of the NOx storage member. Thus, the NOx reduction amount is high in a rich atmosphere, and the sulfur poisoning takes place less-remarkably.
Rh is used in a state where it is loaded on ZrO2 at least. Namely, ZrO2 has a function of sharply improving the steam reforming reaction when it is combined with Rh. In Rh/ZrO2 the loading amount of Rh can preferably be in a range of from 0.1 to 10% by weight, and can optimally be in a range of from 0.5 to 2% by weight. When the loading amount of Rh is less than 0.1% by weight, the NOx reduction capability decreases, when loading it in excess of 10% by weight, the effect saturates and it is unpreferable in terms of costs.
Furthermore, the ratio of Rh/ZrO2 with respect to the porous oxide in the support can preferably be from 5 to 50% by weight in the support. When Rh/ZrO2 is less than 5% by weight, the NOx reduction capability decreases in a rich atmosphere, and when it exceeds 50% by weight, the porous oxide amount is so less that the purifying performance decreases relatively.
By the way, ZrO2 may have a drawback in that it exhibits a lower heat resistance compared with Al2O3, which is often used as a support for a noble metal, so that the specific surface area decreases by the heat in the service as an exhaust gas purifying catalyst, and thereby the dispersibility of the loaded Rh may decrease so that the purifying performance decreases.
Hence, as for a support for Rh, it is preferable to use ZrO2 which is stabilized by an alkaline-earth metal or lanthanum. By using the stabilized ZrO2 support, since the heat resistance is improved so sharply that the highly dispersed state of Rh is maintained, a much higher purifying performance can be obtained even after a durability test.
Moreover, in the exhaust gas purifying catalyst according to the present invention, the porous oxide including TiO2 at least is used together with the aforementioned Rh/ZrO2. As for this porous oxide, although Al2O3, SiO2 zeolite, and so on, can be exemplified, it is preferable to contain Al2O3 in addition to TiO2. Al2O3 can coexist with TiO2 as an independent oxide, or can preferably constitute a composite oxide together with TiO2.
TiO2, as aforementioned, may have a disadvantage in that it reacts with the NOx storage member to form composite oxides (BaTiO3 etc.) so that the NOx reduction capability decreases sharply. On the other hand, when TiO2 coexists with Al2O3, or when a composite oxide of TiO2 and Al2O3 is formed, it has been apparent that they are inhibited from generating the composite oxides of the NOx storage member and them. Therefore, as the porous oxide, when Al2O3 is included in addition to TiO2, it is much better in the heat resistance, and can maintain a high NOx conversion even after a durability test. Moreover, by the existence of Al2O3, a high NOx performance can be obtained even initially.
When TiO2 is mixed as a powder, it is preferred that the particle diameter can fall in a range of from 10 to 1,000 xc3x85. When the particle diameter is less than 10 xc3x85, the particles of BaTiO3, and so on, are coarsened, because the particles as a whole react with the NOx storage member. Moreover, when the particle diameter exceeds 1,000 xc3x85, the effect of TiO2 addition is not revealed, because the surface area of TiO2 decreases. Accordingly, when the particle diameter of TiO2 falls outside the aforementioned range, it is unpreferable because the NOx conversion decreases after a durability test even in either of the cases.
A ratio of Al2O3 to TiO2 can preferably fall in a range of Al2O3/TiO2=30/1 to 1/30. When TiO2 is less than the range, it is difficult to inhibit the sulfur poisoning, and when TiO2 exceeds this range, a sufficient purifying performance cannot be obtained.
Note that the support can further include a porous oxide of good gas adsorbing property, such as SiO2 ZrO2 SiO2xe2x80x94Al2O3, and so on, in addition to Al2O3 and TiO2.
As for the NOx storage member, it is at least one member selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements, as for the alkali metals, it is possible to list lithium, sodium, potassium, rubidium, cesium and francium. Further, the alkaline-earth metals are referred to as the elements of group IIa in the periodic table of the elements, it is possible to list barium, beryllium, magnesium, calcium and strontium. As for the rare-earth elements, it is possible to list scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, and so on.
The loading amount of the NOx storage member can preferably be from 0.01 to 10 mole with respect to 100 g of the support. When the loading amount of the NOx storage member is less than 0.01 mole, a sufficient NOx purifying performance cannot be obtained, when the loading amount of the NOx storage member exceeds 10 mole, the NOx storage member covers the surface of the noble metal so that the purifying performance decreases. Moreover, when the loading amount of the NOx storage member exceeds 10 mole, there arises such a drawback that the granular growth of the noble metal is facilitated.
As for the noble metal, at least one member, which is selected from the group consisting of Pt, Pd and Rh, is used. The loading amount of the noble metal can preferably be from 0.05 to 5% by weight with respect to the support. When the loading amount of the noble metal is less than 0.05% by weight with respect to the support, the NOx purifying performance is low, when it is loaded in excess of 5% by weight, the effect saturates and it is unpreferable in terms of costs.
Note that the coexistence form of the porous oxide including TiO2 at least and Rh/ZrO2 can be constituted by mixing the respective powders uniformly, or a coating layer can be constituted by dividing the coating layer into an upper layer and a lower layer, respectively.
And, in the exhaust gas purifying method according to the present invention, by using the aforementioned exhaust gas purifying catalyst, NOx in a lean exhaust gas, which is generated by burning an air-fuel mixture of an oxygen excess lean atmosphere, is stored in the NOx storage member, and, in a rich exhaust gas, which is generated by burning an air-fuel mixture of from a stoichiometric point to a rich atmosphere, the NOx stored in the NOx storage member are released. And, the released NOx are reduced and purified by H2 which is generated by HC and CO in the exhaust gas and the steam reforming reaction of Rh in Rh/ZrO2.
Moreover, the sulfur oxides in the exhaust gas reacts with the NOx storage member to produce the sulfates or sulfides, the sulfates or sulfides are readily decomposed by H2, which is generated by the steam reforming reaction of Rh in Rh/ZrO2 the NOx storage capability of the NOx storage member recovers quickly, and accordingly the NOx storage capability is furthermore improved over the entire atmosphere of the exhaust gas from the lean atmosphere to the rich atmosphere.
Note that the exhaust gas atmosphere is adjusted so that the time, in which the atmosphere is a lean atmosphere, is dozens of times that of the time, in which the atmosphere is from a stoichiometric point to the rich atmosphere, and it is preferred that the atmosphere is varied from a stoichiometric point to a rich atmosphere in a pulsating manner. When the time, in which the atmosphere is from a stoichiometric point to the rich atmosphere, is shorter than the time, it is difficult to reduce and purify NOx when the time, in which the atmosphere is from a stoichiometric point to a rich atmosphere, is longer than the time, it is unpreferred because the fuel consumption enlarges and the CO2 emission increases.